A magneto-optical recording system uses a recording medium formed by depositing a perpendicularly magnetized film made of a magnetic material on a substrate, and performs recording and reproducing operations in the manner mentioned below.
In order to perform recording, the recording medium is initialized by, for example, a strong external magnetic field, and the direction of magnetization is oriented in one direction (upward or downward direction). Thereafter, a desired recording area is irradiated with a laser beam to increase the temperature of the medium in the area to at least near its Curie point or compensation point so that the coercive force (Hc) in the area becomes zero or substantially zero. Subsequently, an external magnetic field (bias magnetic field) opposite to the initialized magnetization direction is applied to reverse the magnetization direction. When the irradiation of the laser beam is stopped, the recording medium returns to ordinary temperature, and the reversed magnetization is fixed. Thus, information is thermomagnetically recorded.
In order to perform reproduction, the recording medium is irradiated with a linearly polarized laser beam, and information is optically read using such phenomena that the plane of polarization of light reflected from or transmitted through the recording medium is rotated (Kerr magnetic effect and Faraday magnetic effect).
The magneto-optical recording system has been focused as a rewritable large-capacity memory element. As a system for reusing (rewriting) the recording medium, a so-called light-intensity modulation overwritable medium was proposed. The light-intensity modulation overwritable medium enables overwriting by using an exchange-coupled-two-layer film, an initialization magnetic field (Hi) and a recording magnetic field (Hw) and by performing light-intensity modulation. Further, a light-intensity modulation overwritable medium of another type was also proposed. This light-intensity modulation overwritable medium includes an exchanged-coupled-four-layer film, and performs overwriting without using an initializing magnetic field (Hi).
Referring now to FIGS. 16 to 18, the following description will briefly explain the process of light-intensity modulation overwriting using the light-intensity modulation overwritable medium which includes the exchange-coupled four-layer film and requires no initializing magnetic field Hi.
As illustrated in FIG. 16, the light-intensity modulation overwritable medium includes a first magnetic layer 13, a second magnetic layer 14, a third magnetic layer 15, and a fourth magnetic layer 16. The temperature dependence of the coercive forces of these magnetic layers is shown in FIG. 17.
Next, the changes in the magnetization state of each magnetic layer will be explained with reference to FIG. 18. The arrows in FIG. 18 show the magnetization direction of transition metal.
At room temperature, information is recorded depending on whether the magnetization direction of the first magnetic layer 13 is upward "0" (state S71) or downward "1" (state S77). The magnetization of the fourth magnetic layer 16 is always oriented in one direction (an upward direction in FIG. 18), and the magnetization of the second magnetic layer 14 is oriented in the same direction as that of the fourth magnetic layer 16 through the third magnetic layer 15.
Recording is performed by irradiating laser light whose intensity has been modulated to high power or low power while applying the recording magnetic field Hw.
The high power and low power are set so that the medium is heated to a temperature near Curie point Tc2 of the second magnetic layer 14 (state S74) when laser light of high power is irradiated, and heated to a temperature near Curie point Tc1 of the first magnetic layer 13 (state S73) when laser light of low power is irradiated.
Therefore, when the laser light of high power is irradiated, the magnetization of the second magnetic layer 14 is switched to a downward direction by the recording magnetic field Hw (state S75), and copied to the first magnetic layer 13 by an exchange force acting on the interface during a cooling process (state S76). Then, the magnetization of the second magnetic layer 14 is oriented in the same direction as that of the fourth magnetic layer 16 (state S77). As a result, the first magnetic layer 13 shows the downward magnetization direction "1".
On the other hand, when the laser light of low power is irradiated, the magnetization of the second magnetic layer 14 is not switched by the recording magnetic field Hw because its coercive force is stronger than the recording magnetic field Hw (state S73). Similarly to the above case, the magnetization direction of the first magnetic layer 13 is aligned with the magnetization direction of the second magnetic layer 14 by the exchange force acting on the interface during the cooling process (state S72). Therefore, the first magnetic layer 13 shows the upward magnetization direction "0" (state S71).
The laser power used for reproduction is set to a level much lower than the low power for recording.
Hence, the above-mentioned conventional technique uses an exchanged-coupled-four-layer film, and provides a magneto-optical recording medium capable of being overwritable by light-intensity modulation without requiring the initializing magnetic field Hi and of achieving stable recording bits.
In this conventional technique, however, it is necessary to orient the magnetization of the fourth magnetic layer 16 in one direction using a large magnetic field or high laser power before shipping from factories or before recording. Consequently, the conventional technique suffers from such a drawback that the costs of manufacturing the magneto-optical recording medium and a device for recording information on the medium increase.
Moreover, if the direction of the magnetization of the fourth magnetic layer 16 which has been oriented in one direction is disordered for some reasons, light-intensity modulation overwriting cannot be carried out.